574 PHYSIOLOGY. 



which the vasa propria fill in the descent of the elaborated juices. It is 

 clear, however, that the currents of the sap must vary according to the 

 different anatomical disposition of the tissues. 



Summary. We may conclude by repeating that the nutrient 

 fluids in plants follow certain directions, according to the structure 

 and arrangement of the tissues, the place where evaporation is most 

 active, the locality of the sources of nutriment and of growth or 

 other action ; and that as regards the elaborated fluid the movement 

 may be, 1, from the place of formation to that of consumption, or, 

 2, to the store-cells or reservoirs, or, 3, from these latter to the 

 place of consumption. The ascending, descending, or horizontal 

 direction of the currents is therefore a secondary matter. 



To illustrate the movement and transference of nutrient matters, 

 allusion may here be made to the researches of M. A. Gris on the pro- 

 duction and utilization of starch, &c. This observer finds that in winter- 

 time the medullary rays, wood, and pith are all filled with starch-grains. 

 These diminish in spring 1 , but are afterwards replaced during the summer. 

 He concludes from this that there are two special movements of the nutrient 

 substances, as illustrated by their formation in summer and their absorp- 

 tion in the following spring. 



Sect. 6. ELABORATION OF THE FOOD. 



Exhalation of Oxygen. When green plants are placed in water 

 containing dissolved carbonic dioxide, and exposed to sunlight, 

 they give off oxygen gas. 



This may be readily observed in Vattisneria and other submerged green 

 plants grown in glass jars, a continuous stream of bubbles escaping from 

 the plants when standing in the sunshine. The frothy masses of Con- 

 fervse, borne up to the surface of freshwater pools in sunny weather by 

 the entangled bubbles of oxygen, afford another common instance. 



The absorption of carbonic dioxide, and the elimination of oxygen in the 

 case of aquatic plants, and also in that of leathery leaves as in the Cherry 

 Laurel, where tnere are comparatively few stomata, take place chiefly or 

 entirely on the upper surface. 



Where no carbonic dioxide exists, as in boiled or distilled water, no 

 oxygen is liberated. Leafy shoots remaining attached to trees, but en- 

 closed in close glass globes," increase the percentage of oxygen in the globes 

 when exposed to daylight ; and cut shoots with the lower ends placed in 

 water containing carbonic dioxide in solution give off more oxygen than 

 when the lower ends are dipped in water devoid of that gas. 



The oxygen exhaled by leaves &c. is formed at the moment of its 

 liberation; for Confervse, which have no air-passages, and other plants 

 which have had their air-passages exhausted by the air-pump, give off 

 oxygen under the above circumstances. Fragments of loaves perform the 

 Bafne function so long as their organization is uninjured, while the de- 

 struction of the cells by pressure &c. stops the action. The epidermal 

 cells exhale no oxygen. 



